1. Field
The invention relates to an endoprosthesis for a shoulder joint. The endoprosthesis comprises an axial shaft piece for insertion, into the humerus and a neck piece articulated by an articulation member on the shaft piece. The neck piece comprises an axial joint neck alignable with the neck piece to receive a joint head. Fixing means are also provided for fixing the neck piece in a selected alignment and a joint head with a first articulation surface. This articulation surface co-operates with a second artificial or the natural articulation surface on the shoulder. The prosthesis therefore if required also includes a prosthesis part with the artificial second articulation surface.
2. Background Information
A shoulder joint prosthesis is known from EP-A-712 617. It comprises a shaft which is adapted to be anchored in the humeral canal and a cap with a substantially hemispherical profile. The cap can co-operate with the joint socket of the shoulder. In this shoulder joint prosthesis, the shaft comprises an articulation seating with a hemispherical base. A ball is disposed in the articulation seating and comprises means to receive the cap. In addition, locking means are provided, particularly grub screws, which enable the locking of the ball in a specific angular position relative to the axis of the shaft.
This shoulder joint prosthesis, thanks to the ball joint between the shaft and the joint neck, allows stepless regulation of the inclination and rotation of the neck axis. A disadvantage of this prosthesis is that it is relatively complex to make spherical surfaces and the fixing of spherical surfaces with respect to a concentric spherical surface is difficult.
FR 2 773 469 discloses a shoulder joint prosthesis with a shaft piece, a directional member pivotable thereon and carrying a ball cap. In this prosthesis, the metaphysal shaft piece end is provided with a hemispherical recess with a screwthreaded bore in the base thereof. The recess and screwthreaded bore are aligned to a neck axis, the angle of which to the shaft axis is preselected. A pivotable directional member fits in the recess and has a hemispherical surface at its humeral end, said hemispherical surface being concentric with the hemispherical surface of the recess. The directional member has an axial bore with a hemispherical base. A screw with a spherical head is guided with the shank forward into the bore, the screwthreaded portion of the shank being pushed through a conical bore in the hemispherical base of the directional member and screwed into the screwthreaded bore in the shaft piece. The centers of the spherical surfaces of the recess in the shaft piece, inner and outer spherical surfaces on the directional member and the spherical surface of the screw head are situated at a common central point when the screw is tightened. As a result of this construction, the intermediate member is adapted to be pivotable steplessly in all directions relative to the shaft piece and simultaneously rotatable.
A disadvantage of this prosthesis is that the four spherical surfaces must have a common center and therefore have to be made with the maximum precision to ensure a sufficient hold between the spherical surfaces.
DE-U-299 18 589.3 discloses a shoulder joint prosthesis which avoids this problem of extreme precision for the spherical surfaces. This prosthesis has a shaft piece for implanting in the humerus with a shaft head. In the area of the shaft head an articulation surface is formed on which a rotary member is disposed. This is rotatable relatively to the shaft piece about a first axis. A directional member is articulated on the rotary member and extends along a directional axis and is rotatable about a second axis with respect to the rotary member. The second axis extends transversely of the first axis and transversely of the directional axis. The latter is thus steplessly pivotable in all directions. The rotary member and the directional member are each fixable in a selectable position. A head cap is connectable to the shaft piece via the directional member and the rotary member. Articulation or contact surfaces between the shaft piece and the rotary member allow only a relative movement between the shaft piece and the rotary member about their common first axis. Articulation or contact surfaces between the rotary member and the directional member allow only a relative movement between the rotary member and the directional member about the common second axis as rotation center.
A disadvantage of this endoprosthesis is that at least two intermediate members are required between the shaft piece and the cap.